There are already known various constructions of bearings, among them such in which a pressurized fluid is being supplied into the bearing to form a thin supporting film which supports the shaft which is received in the bearing for rotation. Such bearings perform to satisfaction so long as the supply of the pressurized fluid is assured regardless of the rotation of the shaft, that is, so long as the thin supporting film is in existence at any time at which the shaft rotates, including the start-up and wind-down phases of the rotation of the shaft. On the other hand, the operation of such bearings leaves much to be desired when they are being used in applications in which the pressure of the pressurized fluid supplied to the bearing is dependent on the speed of rotation of the shaft that is rotatably supported in the bearing, since then the shaft may run "dry" during certain time periods, such as during the start-up and wind-down phases of rotation of the shaft when the pressure of the pressurized fluid is directly proportional to the speed of rotation of the shaft. This, of course, is very disadvantageous, since such "dry" operation results in an excessive wear of the bearing. Moreover, the bearing offers an excessive frictional resistance to the rotation of the shaft during such "dry" operation, which is especially disadvantageous during the start-up phase, since either the period of time needed to reach the required shaft speed is undesirably long, or the motor driving the shaft will have to be overdimensioned.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a bearing arrangement of the type here under consideration, in which the wear of the bearing material is reduced to a minimum.
Yet another object of the present invention is to design the bearing arrangement of the above type in such a manner as to keep the frisupporting a rotatable shaft used in a system employing a pressurized medium at a pressure which is dependent on the speed of rotation of the shaft, this bearing arrangement including main bearing means that includes a bearing sleeve mounted on a support and surrounding a first portion of the shaft, and means for supplying the pressurized working medium between the first portion of the shaft and the bearing sleeve to fully support the shaft in the bearing sleeve when the pressure of the pressurized working medium exceeds a predetermined value. The bearing arrangement further includes auxiliary bearing means including at least one bearing member which is mounted on the support for displacement into and out of contact with a second portion of the shaft, and means for urging the bearing member into contact with the second portion of the shaft to furnish at least a portion of the required supporting effect when the pressure of the pressurized working medium is below the predetermined value.
It is particularly advantageous when the bearing member is constituted by a split bearing ring which has a conical outer circumferential surface and when there is further provided an outer bearing ring which is stationary and surrounds the split bearing ring, this outer bearing ring having a conical inner ramp surface which is adapted to contact the conical outer circumferential surface of the split ring to act as a guide therefor. Then, the urging means advantageously includes resilient spring means which acts on the split bearing ring in one axial direction of the shaft to cause the outer circumferential surface of the split bearing ring to slide on the inner ramp surface of the outer bearing ring in the sense of reducing the transverse dimensions of the split bearing ring and bringing the inner surface of the split bearing ring into supporting contact with the second portion of the shaft. The pressure of the pressurized working medium may then advantageously be used to counteract the influence of the resilient spring means on the split bearing sleeve and to displace the split bearing ring in the opposite axial direction and out of contact with the second portion of the shaft when the pressure of the pressurized working medium exceeds the predetermined value.
A particular advantage of the bearing arrangement according to the present invention is that the auxiliary bearing means acts as the exclusive bearing means at the commencement of the rotation of the shaft when the pressure of the pressurized working medium is at zero gauge, while the main bearing means and particularly the pressurized working medium supplied thereto acts as the sole bearing means during operation within the normal range of speeds of rotation of the shaft. This advantage is achieved because the auxiliary bearing means, which is advantageously made of a low-friction material, such as polyimide, filled polyimide or filled polytetrafluoroethylene, offers very low frictional resistance to the rotation of the shaft when active, while its supporting action protects the main bearing means which at that time is not protected by the film of the working medium since the latter is at an insufficient or non-existent superatmospheric pressure. On the other hand, only the still lower frictional resistance of the film of the pressurized working medium present in the main bearing means to the rotation of the shaft is effective during the normal operation of the equipment including the shaft when the pressure of the working medium as determined by the speed of rotation of the shaft exceeds the predetermined value, since then the auxiliary bearing means is out of contact with the second portion of the shaft. Thus, the bearing arrangement of the present invention takes advantage of the benefits of both self-lubricating bearings and working fluid lubricated bearings, without incurring the disadvantages of either one of them.